1. Field of the Invention
The present invention broadly relates to a pipe-forming stand and continuous method for making welded steel pipes. It relates particularly to the use of pipe-forming rolls for continuously forming sheet-like steel skelp having different widths and thicknesses into steel pipe sheet material, and into welded seam tubular steel shapes.
2. Description of the Related Art
In general, there have conventionally been two types of processes for making welded steel pipes. They are forge welding and seam welding.
Forge welded pipes are formed from skelp which is continuously heated in a furnace to about 1350.degree. C. to 1400.degree. C. Using forming rolls, the heated skelp is formed into a tubular form having a V-shaped open portion with separate edges. After subjecting both edges of the open portion of the formed skelp to oxygen blowing, thereby further increasing the temperature of the formed skelp edges by oxidation heat, the edges of the V-shaped open portion are pressed and joined using a forge welding roll. In general, a set of two rolls each provided with holes is used for forming the pipe. When the skelp is being formed into a pipe, cooling water is at all times poured onto these rolls to prevent the generated heat from damaging them. Since forge welded steel pipes are red hot at 1350.degree. C. or more, merely pouring cooling water on the forming rolls does not cool them down to a temperature that would prevent forge welding.
Seam welded pipes are formed from skelp continuously formed at an ordinary temperature. A forming device forms the skelp into an open tubular form having a V-shaped open portion, and both exposed edges of the open portion are heated by induction heating or by electrical resistance heating, at high frequency, to a temperature above the melting point. Then usually in successive stages, the ends of the open tubular form are pressed toward each other and are thus joined together by using a squeeze roll. The pipe-forming device ordinarily comprises a large number of separate sets of rolls (such as breakdown rolls for the first forming period, side rolls and/or cage rolls for a subsequent forming period, and fin pass rolls for yet another successive forming period). These rolls gradually curve the skelp from an open form to a closed form in order to eventually form the skelp into a hollow cylindrical pipe.
Although forge welded pipes can be produced rapidly at a speed of 300 m/min, the skelp used to form the forge welded pipes is heated to a high temperature of 1350.degree. C. or more. This causes scales to tend to form at the edges of the skelp, causing the strength of the seam portion of the pipe to be considerably less than the strength of the base. This results in poor surface texture caused by the scales on the pipe surface. Therefore, forge welded pipes cannot be used as high grade steel pipes which must meet the requirements of surface quality and strength reliability of, for example, STK of Japanese Industrial Standards (JIS).
Seam welded pipes, which are produced at ordinary temperature, have good seam quality and surface texture, and are used as high grade steel pipes. However, large weld beads, which are formed at the seam portion by welding and pressing the skelp edges together, must be cut on-line. Accordingly, the pipe-forming speed can only be about 100 m/min at most, thereby resulting in low production speed and efficiency.
We have overcome all such problems by using a new pipe forming method, which may be called a seam welding type method for producing pipes by solid-phase pressure welding. In this method, the skelp is heated to a warm forming temperature (for example, about 600.degree. C.) in which only a small amount of scale is generated on the skelp. Then, as in the case where seam welded pipes are produced, the skelp is formed, and the edges of the skelp are heated to a temperature below the melting point, at which temperature large beads are not formed. Thereafter, the edges of the skelp are physically pressed and joined together.
This method, however, has the following disadvantages. In this method, the skelp, whose temperature falls within the warm forming temperature, is formed using a forming device. When a forming roll contacts the hot skelp, heat is conducted to the roll shaft bearing, causing its temperature to rise excessively and causing poor forming precision. In addition, when the forming roll heats it undergoes thermal expansion and the gap between the upper and lower rolls becomes narrower, making it difficult continuously to form the skelp. To prevent this, it is necessary to subject the rolls to water cooling. Unlike the case where pipes are formed by forge welding, when external water cooling is performed, the cooling water, which is poured onto the skelp, reduces the temperature of the skelp. This makes it difficult to maintain the temperature of the edge portions of the skelp within the temperature range that allows solid-phase pressure welding, just below the melting point, when heating at a high frequency is being performed.
The need to produce a large variety of different steel pipe products has caused a strong demand for efficient production when the volume of production of a particular tubular product is relatively low. To respond to this demand, it is essential to achieve continuous production of pipes with skelp materials of different widths and/or thicknesses. This requires a forming device which, in a breakdown mill, allows the distance between the upper and lower rolls to be changed, and which also allows changing the distance between the divided left-hand and right-hand portions of each of the horizontal upper and lower rolls.
In order to change the distance between the upper and lower horizontal rolls, it is possible to use a mechanism for raising and lowering bearing boxes which support a roll shaft.
In order to change the distance between the left-hand and right-hand portions of a divided roll, a mechanism may be used which uses a hydraulic arbor, capable of increasing and decreasing the diameter thereof, as a shaft. The diameter of the shaft is decreased in order to allow the divided roll to move freely in the axial direction for positioning. Then, the diameter of the shaft is increased in order to lock the divided roll. However, the hydraulic arbor has a hydraulic mechanism built therein, making it difficult additionally to dispose a water cooling mechanism therein.
Accordingly, when the volume of production is relative low and a wide variety of tubular products are to be made, the seam welding method of producing pipes by solid phase pressure welding cannot be used along with a conventional forming stand using rolls.